Maximum possible value

Algebra Level 3

If a + b + c = 400 a+b+c=400 , where a , b and c a,b \text{ and} \ c are non-negative numbers, then what is the maximum possible value of

2 a + b + 2 b + c + 2 c + a = ? \large \sqrt{2a+b} + \sqrt{2b+c}+ \sqrt{2c+a}=?

60 59 50 58 57

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Chew-Seong Cheong
Aug 11, 2017

For a , b , c > 0 a, b, c > 0 , we can apply the Cauchy-Schwarz inequality .

( 2 a + b + 2 b + c + 2 c + a ) 2 ( 1 2 + 1 2 + 1 2 ) ( ( 2 a + b ) 2 + ( 2 b + c ) 2 + ( 2 c + a ) 2 ) 3 ( 2 a + b + 2 b + c + 2 c + a ) = 9 ( a + b + c ) = 3600 2 a + b + 2 b + c + 2 c + a 60 \begin{aligned} \left(\sqrt{2a+b} + \sqrt{2b+c} + \sqrt{2c+a} \right)^2 & \le \left(1^2+1^2+1^2\right)\left((\sqrt{2a+b})^2 + (\sqrt{2b+c})^2 + (\sqrt{2c+a})^2 \right) \\ & \le 3 \left(2a+b + 2b+c + 2c+a \right) = 9(a+b+c) = 3600 \\ \implies \sqrt{2a+b} + \sqrt{2b+c} + \sqrt{2c+a} & \le \boxed{60} \end{aligned}

Thank you for sharing your solution.

Hana Wehbi - 3 years, 10 months ago
Genis Dude
Aug 12, 2017

Due to symmetry, a=b=c.

By using that we get 60,which is the maximum

value in the option.

Therefore, 60 is the answer.

Your logic is wrong. The min/max of a function does not always occur when all its variables are equal. See Inequalities with strange equality conditions .

Pi Han Goh - 3 years, 10 months ago

Log in to reply

But the maximum value in the option is 60.

genis dude - 3 years, 10 months ago

Log in to reply

What if 60 isn't given as an answer?

Steven Jim - 3 years, 9 months ago

Log in to reply

@Steven Jim Then I would view the solution. XD

genis dude - 3 years, 9 months ago

@Steven Jim When it comes to competitive objective exams, this method is good

genis dude - 3 years, 9 months ago

Log in to reply

@Genis Dude "When it comes to competitive objective exams" doesn't mean you should spam the answer that way. Considering that Brilliant is a place to "learn", it's advisable to give good solutions. Yes, the method is good, but what if there are no "possible answers" for you to choose?

Steven Jim - 3 years, 9 months ago

Log in to reply

@Steven Jim Then, I would try and I'm not spamming, this is a good method used by many maths nerds.

genis dude - 3 years, 9 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...